Methods

Sample Population

This cross-sectional analysis included definitive urine drug testing (UDT) results obtained via liquid chromatography-tandem mass spectrometry (LC-MS/MS) on patient specimens collected in multiple health care specialties across the U.S. The LC-MS/MS testing method is a laboratory-developed test with performance characteristics determined by Millennium Health, San Diego, California, which is certified by the Clinical Laboratory Improvement Amendments and accredited by the College of American Pathologists for high-complexity testing. Included specimens were collected from patients aged 18 years or older between April 12, 2023, and April 11, 2024. All specimens had positive UDT results for fentanyl and had ordered and valid test results for xylazine. Over 50,000 specimens from more than 23,000 unique patients were selected for analysis. Any patient with more than 12 specimens during the analyzed period was excluded from the analysis.

The following drug categories were evaluated (analytes tested in parentheses): xylazine (xylazine, 4-OH-xylazine) and fentanyl (fentanyl, norfentanyl). If any parent drug or metabolite within a given drug category was ordered and detected, the result for that drug category was considered positive. Specimens with reported prescriptions for fentanyl were excluded from analysis; xylazine is not FDA-approved for use in humans. Xylazine detection was evaluated in fentanyl-positive specimens because previous research has shown that over 99% of all xylazine positive specimens were also positive for fentanyl.⁶

Statistical Methods

UDT positivity rates were modeled using binomial generalized estimating equations (GEE). GEE is a robust estimator for longitudinal data when the correlation structure of observations is unknown and is an alternative to standard generalized linear mixed effect models. An `exchangeable` correlation structure was used for model fitting, and sandwich robust standard errors were calculated to account for the clustered nature of patient data (R geeglm function). Model 1 was designed to compare xylazine detection between the first 6-months (Apr 12, 2023 – Oct 10, 2023) and second 6-months (Oct 11, 2023 – Apr 11, 2024) of the study period for each U.S. Census Division. For this model, an interaction between time-period and U.S. Census Division was included. Model 2 was designed to evaluate state-level xylazine detection for the entire 12-month study period and included a simple additive effect for patient state of residence. Sex, age (discrete variable), specialty of the referring clinician, and payer were treated as covariates in all models. Adjusted Positivity Rates (Least Square Mean probabilities) and adjusted odds ratios (aOR) were estimated along with robust 95% confidence interval (CI) values. Percent change in Model 1 was calculated as: (2nd 6-month adjusted positivity – 1st 6-month adjusted positivity) / 1st 6-month adjusted positivity × 100. Significant changes for each U.S. Census Division were based on non-overlapping 95% CI values for 1st and 2nd 6-month positivity rates. States included in the analysis were restricted to those with 50 or more fentanyl-positive results for accurate xylazine positivity rate determination.

References

1. United States Food and Drug Administration (FDA). FDA Alerts Health Care Professionals of Risks to Patients Exposed to Xylazine in Illicit Drugs. https://www.fda.gov/drugs/drug-safety-and-availability/fda-alerts-healthcare-professionals-risks-patients-exposed-xylazine-illicit-drugs. Published November 8, 2022. Accessed June 3, 2024.
2. Drug Enforcement Administration (DEA). The Growing Threat of Xylazine and Its Mixture with Illicit Drugs. https://www.dea.gov/documents/2022/2022-12/2022-12-21/growing-threat-xylazine-and-its-mixture-illicit-drugs. Published October 2022. Accessed June 3, 2024.
3. Spencer MR, Cisewski JA, Warner M, Garnett MF. Drug overdose deaths involving xylazine: United States, 2018–2021. Vital Statistics Rapid Release; no 30. Hyattsville, MD:National Center for Health Statistics. 2023. DOI: https://dx.doi.org/10.15620/cdc:129519.
4. The Center for Forensic Science Research and Education (CFSRE). Xylazine: A Toxic Adulterant Found in Illicit Street Drugs. https://www.cfsre.org/nps-discovery/public-alerts/xylazine-a-toxic-adulterant-found-in-illicit-street-drugs Published October 2020.
5. Biden-Harris Administration Designates Fentanyl Combined with Xylazine as an Emerging Threat to the United States.;2023. Available at: https://www.whitehouse.gov/ondcp/briefing-room/2023/04/12/biden-harris-administration-designates-fentanyl-combined-with-xylazine-as-an-emerging-threat-to-the-united-states/. Accessed June 3, 2024.
6. Millennium Health LLC. Zeroing in on Xylazine: The First 100 Days After U.S. Emerging Threat Designation. Millennium Health Signals ReportTM, Voume 5.1. /signalsreport/. Published September, 2023.